1. Field of the Invention
The present invention relates to a dressing apparatus for dressing a polishing surface of a polishing table used for polishing a workpiece such as semiconductor wafer in a polishing apparatus, and a polishing apparatus, having such a dressing apparatus, for polishing a workpiece such as a semiconductor wafer to a flat mirror finish.
2. Description of the Related Art
Recently, semiconductor device have become more integrated, and structure of semiconductor elements has become more complicated. In addition, as the number of layers in multilayer interconnections used for a logical system has been increased, irregularities of a surface of a semiconductor device are increased, so that a step height on the surface of the semiconductor device becomes larger. This is because in manufacturing semiconductor devices, a process for forming a thin film is performed and a micromachining process such as patterning or formation of holes is performed, and then a process for forming a subsequent film is performed, and these processes are repeated many times.
When the irregularities of the surface of the semiconductor device are increased, the following problems arise. Thickness of a film formed in a portion having a step is relatively small. An open circuit is caused by disconnection of interconnections, or a short circuit is caused by insufficient insulation between layers. As a result, good products cannot be obtained, and yield is reduced. Further, even if a semiconductor device initially works normally, reliability of the semiconductor device is lowered after a long-term use.
Another problem is caused with regard to a lithography process by irregularities of a surface of the semiconductor device. Specifically, at a time of exposure during the lithography process, if an irradiation surface has irregularities, then a lens unit of an exposure system is locally unfocused. Therefore, if the irregularities of the surface of the semiconductor device are increased, then it is difficult to form a fine pattern on the semiconductor device.
Thus, during a manufacturing process of a semiconductor device, it is increasingly important to planarize a surface of the semiconductor device. The most important one of planarizing technologies is chemical mechanical polishing (CMP). During chemical mechanical polishing, in which a polishing apparatus is used, while a polishing liquid containing abrasive particles such as silica (SiO2) therein is supplied onto a polishing surface such as a polishing pad, a substrate such as a semiconductor wafer is brought into sliding contact with the polishing surface, thereby polishing a surface of the substrate.
Conventionally, as shown in FIG. 17, such a polishing apparatus has a polishing table 102 having a polishing cloth (polishing pad) 100 attached to an upper surface thereof, and a top ring 104 for holding a substrate W, such as a semiconductor wafer, and pressing the substrate W against the polishing cloth 100 on the polishing table 102. A polishing liquid containing abrasive particles is supplied form a nozzle 106 onto the polishing cloth 100 and retained on the polishing cloth 100. The polishing cloth 100 on the polishing table 102 constitutes a polishing surface of the polishing table. During operation, the top ring 104 exerts a certain pressure, and a surface of the substrate W held against the polishing surface of the polishing table 102 is therefore polished to a flat mirror finish while the top ring 104 and the polishing table 102 are rotating. The polishing liquid comprises abrasive particles such as silica particles, and chemical solution such as alkali solution in which the abrasive particles are suspended. Thus, the substrate W is chemically and mechanically polished by a combination of a mechanical polishing action of abrasive particles in the polishing liquid and a chemical polishing action of chemical solution in the polishing liquid.
When a polishing process is finished, polishing capability of the polishing cloth 100 is gradually deteriorated due to a deposition of abrasive particles and ground-off particles removed from the substrate, and due to changes in characteristics of a surface of the polishing cloth. Therefore, if the same polishing cloth is used to repeatedly polish substrates W, a polishing rate of the polishing apparatus is lowered, and polished substrates tend to suffer polishing irregularities. Therefore, it has been customary to condition the polishing cloth according to a process called xe2x80x9cdressingxe2x80x9d for recovering the surface of the polishing cloth before, after, or during polishing.
In order to dress the surface of the polishing cloth 100 which has been deteriorated by polishing, a dressing apparatus 108 having a dressing surface is provided adjacent to the polishing table 102. In operation, the dressing surface of the dressing apparatus 108 is pressed against the polishing surface of the polishing table 102, and the dressing surface and the polishing table 102 are rotated relatively to each other for thereby bringing the dressing surface into sliding contact with the polishing surface. Thus, polishing liquid and ground-off particles attached to the polishing surface are removed, and planalization and regeneration of the polishing surface are conducted.
In order to primarily remove the polishing liquid and the ground-off particles from the polishing surface, a dressing apparatus having a dressing surface composed of a nylon brush is mainly used. In order to primarily planalize the polishing surface by slightly scraping the polishing surface, a dressing apparatus having a diamond dresser is mainly used. Uniformity of the polishing surface which has been dressed greatly affects polishing precision of a workpiece (substrate).
However, the above-mentioned polishing apparatus has the following problems:
A first problem is that in case of polishing a substrate by a polishing table which rotates about its own axis, there is no relative movement between a polishing surface and the substrate relative to a rotational center of the polishing table, and hence the substrate is polished on an area of the polishing surface spaced from the rotational center of the polishing table. Therefore, a diameter of the polishing table should be at least two times a diameter of the substrate. Thus, size of the polishing apparatus becomes large, whereby a large installation space of the polishing apparatus is required, and cost of facilities is high. This drawback is becoming significant with increasing diameters of substrates.
A second problem is presented by a polishing cloth made of material having elasticity, such as urethane. In general, a device pattern on an upper surface of a semiconductor wafer (substrate) has various irregularities having various dimensions and steps, and is composed of different material. When the semiconductor wafer having step-like irregularities is planarized by a polishing cloth having elasticity, not only raised regions but also depressed regions are polished, and hence a large amount of material is removed from the semiconductor wafer and a long period of time is required until the semiconductor wafer is planarized. Thus, an operation cost associated with such a polishing process is increased, and irregularities of a polished surface of the semiconductor wafer are difficult to be eliminated, with a result that a high flatness of the polished surface cannot be obtained. Further, regions on which microscopic irregularities are concentrated are polished at a high polishing rate, and regions on which macroscopic irregularities exist are polished at a low polishing rate. Thus, a large undulation is formed on the polished surface of the semiconductor wafer.
A third problem is presented by operation costs associated with a polishing process and environmental pollution. In order to polish a semiconductor wafer to a high degree of flatness, a polishing liquid needs to be supplied abundantly onto a polishing cloth. However, supplied polishing liquid is discharged from the polishing cloth at a high rate without being used during an actual polishing process. This leads to a high operating cost associated with the polishing process because the polishing liquid is expensive. Further, since the polishing liquid contains a large amount of abrasive particles such as silica particles, and may contain chemicals such as acids or alkalis to thus form slurry-like material, it is necessary to treat waste liquid discharged from the polishing process for thereby preventing environmental pollution. This also leads to a high operating cost associated with the polishing process.
In order to solve the first problem, it is conceivable that the polishing apparatus incorporates a polishing table which makes a circulative translational motion (scroll motion) along a circle having a certain radius. In this case, every point on a polishing surface of the polishing table makes the same motion, and hence a diameter of the polishing surface on the polishing table may be equal to a dimension obtained by adding twice a radius of gyration of the polishing table to a diameter of a semiconductor wafer (substrate).
Thus, the polishing apparatus may be small in size such that installation space of the polishing apparatus may be reduced to lower overall costs, including manufacturing costs of the polishing apparatus, operating costs in a plant and cost of equipment.
In order to solve the second and third problems, it is conceivable to polish semiconductor wafers (substrates) by using an abrading plate. The abrading plate comprises abrasive particles such as silica particles and a binder for binding the abrasive particles, and is flat. The abrading plate may be called a fixed abrasive. The abrading plate is attached to an upper surface of a polishing table, and a semiconductor wafer held by a top ring is pressed against the abrading plate under a certain pressure and brought into sliding contact with the abrading plate. With sliding contact between the abrading plate and the semiconductor wafer, the semiconductor wafer is polished while the binder is broken or dissolved to thus generate fresh freed abrasive particles.
According to the above polishing process, the abrading plate is harder than a polishing cloth and has less elastic deformation than does a polishing cloth, and hence only raised regions on a semiconductor wafer are polished and undulation of a polished surface of the semiconductor wafer is prevented from being formed. Further, since a slurry-like polishing liquid containing a large amount of abrasive particles is not used, an amount of wafers discharged from the polishing process, and required to be treated, is greatly reduced, and hence an operating cost is reduced and environmental protection is easily carried out. Since a polishing liquid containing abrasive particles is not used, equipment for supplying such polishing liquid is not required.
In a case where an abrading plate is attached to a polishing table which makes a circulative translational motion (scroll motion), and a substrate is polished by the abrading plate, a polishing surface of the abrading plate includes a central region which is always in contact with the substrate while being polished, a peripheral region which is always not in contact with the substrate while being polished, and an intermediate region which is brought into contact with or out of contact with the substrate while being polished. As a result, as shown in FIG. 18, a surface of abrading plate 110, i.e. a polishing surface 110a, has a depressed region. That is, a central region A of the polishing surface suffers a large abrasion loss, a peripheral region C suffers hardly any abrasion loss, and an intermediate region B suffers an inclined abrasion loss. Even if a substrate (semiconductor wafer) continues to be polished by the polishing surface shown in FIG. 18, the substrate cannot be planarized. Thus, it is necessary to dress the polishing surface of the abrading place.
In such a case, if dressing of the polishing surface is conducted by a dressing tool having a circular dressing surface or an annular dressing surface smaller than the polishing surface, as in the case of conventional dressing process, then the polishing surface of the abrading plate having irregularities is locally dressed, and hence it is difficult to planarize an entire area of the polishing surface. These circumstances hold true for a dressing process of a polishing surface composed of a polishing cloth attached to a polishing table which makes a circulative transnational motion (scroll motion).
It is therefore an object of the present invention to provide a dressing apparatus which can easily and reliably planarize a polishing surface, having irregularities, on a polishing table and regenerate the polishing surface efficiently.
Another object is to provide a dressing apparatus which can dress a polishing surface of a polishing table, which makes a circulative transnational motion (scroll motion or circulative orbital motion) and has an advantage of a small installation space, by a dresser which requires a small installation space, and can increase a processing capability of the polishing table per unit installation area.
According to a first aspect of the present invention, there is provided a dressing apparatus for dressing a polishing surface of a polishing table for polishing a surface of a workpiece. The dressing apparatus comprises a dresser having an elongate dressing surface for dressing the polishing surface. The dressing surface has a flat surface which contacts the polishing surface, and one of a tapered surface extending from the flat surface and inclined so as to be directed away from the polishing surface and a curved surface extending from the flat surface and curved so as to be directed away from the polishing surface.
According to a second aspect of the present invention, there is provided a dressing apparatus for dressing a polishing surface of a polishing table for polishing a surface of a workpiece. The dressing apparatus comprises a dresser having an elongate dressing surface for dressing the polishing surface, wherein the dressing surface comprising a circulate arc surface.
According to a first and second aspects of the present invention, because a boundary portion between a contact portion and a non-contact portion of the dressing surface and the polishing surface has a smooth shape, a stick-slip caused between the dressing surface and the polishing surface can be decreased, and the dresser can be smoothly moved. Thus, generation of vibration of the dresser can be suppressed.
According to the present invention, a long side of the dressing surface has a dimension larger than that of a moving area of the polishing surface of the polishing table, and the dresser is movable along the polishing surface via a horizontally moving mechanism. Thus, an entire area of the polishing surface can be dressed by bringing the dressing surface into contact with the polishing surface and moving the dressing surface. Therefore, even if the polishing surface has local irregularities, the entire area of the polishing surface can be reliably planalized, and the polishing surface can be efficiently and uniformly regenerated.
Further, the long side of the dressing surface must have a dimension equal to or larger than that of the moving area of the polishing table, i.e. a dimension obtained by adding a scroll diameter to a diameter of the polishing table. However, a short side of the dressing surface may be as small as possible, provided that such is permitted by dressing conditions. Thus, a rectangular dresser can save installation space, compared with a circular dresser.
The horizontally moving mechanism may comprise a translation mechanism for causing the dresser to perform a translation along the polishing surface at a constant velocity. With this structure, relative vectors on a contact surface between the dressing surface and the polishing surface, which makes a scrolling motion, are equalized over an entire polishing surface, and contact time between the dressing surface and the polishing surface is equalized over the entire polishing surface, and hence uniform dressing can be performed.
According to a third aspect of the present invention, there is provided a dressing apparatus for dressing a polishing surface of a polishing table for polishing a surface of a workpiece. The dressing apparatus comprises a dresser having a dressing surface for dressing the polishing surface, and a controller for controlling a pressing force for pressing the dresser against the polishing surface such that when a contact area between the dressing surface of the dresser and the polishing surface is changed by relative movement between the dresser and the polishing table during dressing, the pressing force is changed by the controller according to the contact area.
According to the present invention, because a contact area between the dressing surface and the polishing surface is changed by relative movement between the dresser and the polishing table during dressing, and a pressing force of the dresser applied to the polishing table (pressing force applied to an entire dressing surface) is changed by the controller according to the contact area, a pressing force for pressing the dressing surface against the polishing surface (pressure applied to the polishing surface per unit area) can be equalized over an entire polishing surface. Thus, an amount of a material removed from the polishing surface on the polishing table can be uniformized over the entire polishing surface.
According to a fourth aspect of the present invention, there is provided a dressing apparatus for dressing a polishing surface of a polishing table for polishing a surface of a workpiece. The dressing apparatus comprises a dresser having a dressing surface for dressing the polishing surface. The dressing surface has a flat surface which contacts the polishing surface, and one of a tapered surface extending from the flat surface and inclined so as to be directed away from the polishing surface and a curved surface extending from the flat surface and curved so as to be directed away from the polishing surface, and the dresser performs no rotational motion about its own axis during dressing.
According to fifth aspect of the present invention, there is provided a dressing apparatus for dressing a polishing surface of a polishing table for polishing a surface of a workpiece. The dressing apparatus comprises a dresser having a dressing surface for dressing the polishing surface. The dressing surface comprises a circular arc surface, wherein the dresser performs no rotational motion about its own axis during dressing.
In a preferred aspect of the present invention, a dresser cleaning container is provided to clean the dressing surface of the dresser. If the dresser cleaning container is of an elongate shape so as to correspond to a shape of a rectangular dresser, the dresser cleaning container can save installation space. Further, foreign matter attached to the dressing surface, or fragments of the dresser element such as diamond particles, are removed from the dresser, thereby eliminating harmful influence of the polishing surface caused by such fragments and foreign matter.
According to a sixth aspect of the present invention, there is provided a polishing apparatus for polishing surface of a workpiece. The polishing apparatus comprises a polishing table having a polishing surface, and a dresser having a dressing surface for dressing the polishing surface. The dresser is movable along the polishing surface via a moving mechanism, and a shape of the polishing surface is arranged such that a contact area between the dressing surface of the dresser and the polishing surface is not changed when the dresser is moved by the moving mechanism.
According to the present invention, a contact area between the dressing surface of the dresser and the polishing surface is not changed over an entire area where the dresser moves. Thus, a pressing force for pressing the dresser against the polishing surface can be constant, irrespective of a position of the dresser. Shape and size of the polishing surface are set such that the polishing surface is contained in a locus described by an outer periphery of the dresser. As an example of a shape of the polishing surface, the polishing surface is generally rectangular, and a dimension of at least one side of the generally rectangular polishing surface is shorter than a dimension of a long side of the dresser. A moving direction of the dresser is perpendicular to the at least one side of the polishing surface and a moving distance of the dresser is shorter than a dimension of another side of the polishing surface.
According to a seventh aspect of the present invention, there is provided a polishing apparatus for polishing a surface of a workpiece, comprising: a polishing table having a polishing surface; a workpiece holder for holding a workpiece; a pressing device for pressing the workpiece held by the workpiece holder against the polishing surface; and a dressing apparatus for dressing the polishing surface. The dressing apparatus includes the dresser of the above first through fifth aspects of the present invention.